Tag: architecture

I’ve been talking to a lot of line-of-business developers lately. Most have adopted newer technologies like Entity Framework, but many are still working with strictly layered application designs. They’ll have POCO entities, DbContexts, DbSets, and all the modern goodness EF brings. Then they smash it all down into a data access layer, hiding EF behind several layers of abstraction. As a result, these applications can’t leverage EF’s best features, and the entire design becomes very cumbersome.

I blame n-tier architectural thinking!

Pancakes:

Most senior .Net developers cut their teeth on .net during a time when n-tier was a pervasive discussion across the industry. Every book, article and classroom lecture about web application design included a long discussion on n-tier fundamentals. The n-tier hype faded years ago, replaced by more realistic lego-block approaches, but n-tier inspired conceptualizations still exert a strong influence over modern designs.

I call it pancake thinking — conceptualizing the logical arrangement of an application as a series of horizontal layers, each with distinct, non-overlapping areas of responsibility. Pancake thinking pins Entity Framework as a DAL technology –just another means to get data in and out of a database –a fundamental misunderstanding of EF’s intended role.

Here is a diagram of a full blown n-tier approach applied to an ASP.NET MVC application using Entity Framework:

Note that all of entity framework is buried at the bottom. An object mapper is probably transforming EF’s entities into business objects. So, by the time information leaves the DAL, EF has been beaten completely out of it. EF acts only as a modernized version of traditional ADO.NET objects. EF is a better experience for DAL developers, but it doesn’t add value for those writing code further up the stack.

I don’t see as many strictly layered designs like this in the wild, though I have come across a few recently. Most of the current literature around EF advocates a somewhat hybrid approach, like this:

In this design, EF’s POCOs roam around the business and presentation layers like free-range chickens, but DbContext and DbSets are still being tortured in the basement.

What I dislike about this design is that EF has been turned on its head. The DbContext should be at the top of the stack, acting as the entry point through which you interact with entities.

So, how should a modern EF driven application be designed?

EF’s POCO entities, along with DbContext and DbSets, work best with a behavior driven approach to application design. You design around the business-behaviors of your application, largely without concern for what the database looks like. Instead of a horizontally segmented architecture, EF becomes a pervasive framework used throughout the entire application at all levels. If you are worried about mixing “data-logic” with “business-logic”, then you are still thinking about pancakes. Stop it!

Here is another diagram, this time letting Entity Framework do it’s thing, without any unnecessary abstractions:

Notice first, how much simpler the design becomes. We’ve elevated EF to a true business framework, and we’ve eliminated tons of abstractions in the process. We still have n-tier going on, so relax! We just didn’t logically split data and business code like you might be used to.

This is a domain-model architecture, and borrows some of the general ideas of “domain driven design” (DDD). If you are a DDD purist, please do not write me hate mail. What I’m proposing here is not intended to be a true DDD design. If you are interested in DDD in depth, check out Vaughn Vernon’s site. For discussions on practical DDD with .Net and EF, Vaughn’s TechEd presentations are a must watch.

To understand how this design works, let’s explore the three main concepts behind Entity Framework.

EF Entity Model:

The heart of EF is the entity model. At its simplest, the model is just a bunch of data transfer objects (DTOs) –together they form an in-memory model mirroring the database’s structure. If you generate a model from an existing database, you get this shape. This is also what happens if you design a code-first model using the same thinking you’d use to design a relational database. This “in-memory database” viewpoint is how most n-tier applications tend to use EF models.

That simplistic approach doesn’t leverage EF’s power and flexibility. It should be a true business domain model, or something close. Your entities contain whatever code is appropriate for their business function, and they are organized to best fit the business requirements.

The nice thing about a business-oriented model is that code operating against the entities feels very natural to object oriented programmers. You don’t concern yourself with how the actual persistence is done; EF takes care it for you. This is exactly the level of abstraction n-tier designs strive for, but EF gives you the same result without the rigid, horizontal layering.

Persistence mapping:

The logical entity design should be based on business behavior, but the actual code implementation does require you to understand how EF handles persistence.

Persistence details do place some constraints on the kinds of OOP acrobatics you can employ in your model, so you need to be aware of how it works. Overall though, the constraints are mild, and shouldn’t keep you from an implementation that remains true to the intent of the business-centric design.

Many properties on your entities will need to be persisted to the database. Others may exist only to support runtime behaviors, but aren’t persisted. To figure out how, or if, properties map to the database, EF uses a combination of conventions and attribute annotations. EF doesn’t care about your entity’s other methods, fields, events, delegates, etc. so you are free to implement whatever business code you need.

EF does a good job of automatically inferring much of a model’s mapping from code conventions alone. If you use the conventions appropriately you can get a head-start on your persistence mappings –no code needed. For properties that need more explicit definitions, you use attributes to tell EF how to interpreted them.

For really advanced cases, you can hook into the model builder’s fluent API. This powerful tool lets you define tricky mappings that attributes and conventions alone can’t describe fully. If your model is significantly dissimilar from your database’s structure, you may spend a lot of time getting to know the model builder –but it’s easy to use, and amazingly powerful.

While you will need to understand EF persistence issues, you only need to concern yourself with them when you implement the entity model. For code using that model, these details are highly transparent –as they should be.

Repositories and Business Services:

The final piece of EF is the part so many people insist on hiding –the DbContext and DbSets. If you are thinking in pancakes, the DbContext seems like a hub for accessing the database. N-tier principals have trained you to hide data access from other code, at all costs!

Typically, n-tier type abstractions take the form of custom repositories layered on top of entity framework’s objects. Only the repositories may instantiate and use a DbContext, while everything at higher layers must go through the repositories.

A service or unit-of-work pattern is usually layered on top of the custom repositories too. The service manages the repositories, while the repositories manage EF’s DbContext and DbSets.

If you’ve ever tried to layer an n-tier application on EF like this, you probably found yourself fighting EF all over the place. This abstraction is the source of your pain.

An EF DbContext is already an implementation of a unit-of-work design pattern. The DbSet is a generic repository pattern. So you’ve just been layering custom unit-of-work and repositories over top of EF’s unit-of-work and repositories. That extra abstraction doesn’t add much value, but it sure adds a lot of complexity.

Ideally, the DbContext should be a root business service. The most important thing to understand is that this belongs at the top of the business layer, not buried under it.

Your entities directly contain the internal business logic appropriate to enforce their behaviors. Similarly, a DbSet is where you put business logic that operates against the set of an entity type. Anything that you’d normally put in custom repositories can be added to the real DbSet instead through extension methods.

Extension methods let you extend a DbSet on the fly. They are fantastic for dealing with business context specific concerns, and you can have a different set of extension methods for each of your business contexts. The extension methods can be arranged by namespace, and can also be defined in assemblies higher in the stack –in the latter case, the extension may have access to dependencies within the higher layer assembly that would not be appropriate to couple directly to your business layer. For example, an extension method in an asp.net web application can depend on HttpContext, but you would never want to create a dependency like that directly in the business domain. Calling code can just chose which extensions are appropriate, and import/use those namespaces, while ignoring extension methods from other contexts.

For cross-cutting concerns that span multiple entity types, you can extend the DbContext itself. A common approach is to have multiple concrete roots for each of your business contexts. The business specific roots will inherits a common DbContext base class. The base class contains EF specific stuff. Factory and adapter patterns often appear in relation to these roots as well, but the key concept is that each top-level service derives from a real DbContext… your calling code has all the LINQ and EF goodness at its disposal.

If you embrace EF’s DbContext as a top-level business service, either directly or through inheritance, then you will find EF can be a very pleasant experience. You are able to leverage its full power at all layers of your application, and friction with EF’s internals disappear. Using custom abstractions of your own, it is hard to reach this level of fluidity.

The Domain Model you’ve read about online:

If you go online and read recent articles about ASP.NET application designs, you’ll find many advocates of domain model designs. This would be great, except that most of them still argue for custom unit-of-work and repository patterns.

The only difference between these designs, and the hybrid n-tier layering design I described before, is that the abstractions here are a true part of the business layer, and are placed at, or near, the top of the stack.

While these designs are superior to pancake models, I find the additional custom abstraction is largely unnecessary, adds little value, and usually creates the same kinds of friction you see in the n-tier approaches.

The reason for the extra layer of abstraction seems to have two sources. Partially it comes from that legacy n-tier thinking being applied, incorrectly, to a domain model. Even though it avoids full layering, the desire for more abstraction still comes from the designer’s inner-pancake.

The bigger force advocating for extra abstractions comes from the Test Driven Development crowd. Early versions of EF were completely hostile to testing. It took insane OOP acrobatics and deep abstractions even to get something vaguely testable.

In EF 4.1, code-first was introduced. It brought us the first versions of DbContext and DbSets, which were fairly friendly towards unit testing. Still though, dependency injection issues usually made an extra layer of abstraction appealing. These middle-versions of EF are where the design I’ve just diagrammed came from.

In current EF versions (6 or higher), DbContext and DbSets are now pervasive throughout all of EF. You can use them with model-first, database-first, and code-first approaches. You can also use them with POCOs, or with diagram generated entities (which are still POCOs in the end). On the testability front, EF has added numerous features to make native EF objects easily testable without requiring these layers of custom abstraction.

You can, through a bit of experimentation, learn how to write great unit tests directly against concrete instances of DbContext and DbSet –without any runtime dependency on the physical database.

How to achieve that level of testability in your model is a topic for another post, but trust me… you don’t need custom repositories for testing EF anymore. All you need is to be smart about how you compose your code, and maybe a little help from a mocking framework here and there.

I’ve kept most of this discussion pretty high-level. Hopefully it will help expand how you view EF based application designs. With some additional research, you should be able to take these ideas and turn them into real code that’s relevant to your own business domain.